Motors and Generators

Three-PhaseInductionMotorStarter-S579462Ex1.pdf

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Exercise 1 – DOL Starters and Soft Starters  Procedure Outline

The Procedure is divided into the following sections:

 Setup and connections  Starting an induction motor using a DOL starter  Soft starter setup and settings  Starting an induction motor using a soft starter  DOL starter versus soft starter

High voltages are present in this laboratory exercise. Do not make or modify any banana jack connections with the power on unless otherwise specified.

Setup and connections

In this part of the exercise, you will connect your equipment as a DOL starter, configure parameters in LVDAC-EMS so that the Four-Quadrant Dynamometer/Power Supply emulates a flywheel, and set the Oscilloscope to observe the motor start-up current, torque, and speed.

1. Refer to the Equipment Utilization Chart in Appendix A for a list of the equipment required for this exercise.

Install the equipment in the Workstation.

Before coupling rotating machines, make absolutely sure that power is turned off to prevent any machine from starting inadvertently.

Mechanically couple the Four-Pole Squirrel Cage Induction Motor to the Four-Quadrant Dynamometer/Power Supply using a timing belt.

2. Make sure that the ac and dc power switches on the Power Supply are set to the O (off) position and then connect the Power Supply to a three-phase ac power outlet.

Make sure that the main power switch on the Four-Quadrant Dynamometer/ Power Supply is set to the O (off) position and then connect the Power Input to an ac power outlet.

Connect the Power Input of the Data Acquisition and Control Interface (DACI) to a 24 V ac power supply.

Connect the Low Power Input of the Power Thyristors module to the Power Input of the Data Acquisition and Control Interface. Turn the 24 V ac power supply on.

PROCEDURE OUTLINE

PROCEDURE

Exercise 1 – DOL Starters and Soft Starters  Procedure

24 © Festo Didactic 88197-00

3. Connect the USB port of the Data Acquisition and Control Interface to a USB port of the host computer.

Connect the USB port of the Four-Quadrant Dynamometer/ Power Supply to a USB port of the host computer.

4. Turn the Four-Quadrant Dynamometer/Power Supply on and then set the Operating Mode switch to Dynamometer.

5. Turn the host computer on and start the LVDAC-EMS software.

In the LVDAC-EMS Start-Up window, make sure that the Data Acquisition and Control Interface and the Four-Quadrant Dynamometer/Power Supply are detected. Make sure the Computer-Based Instrumentation and Thyristor Control functions are available for the Data Acquisition and Control Interface module. Also, select the network voltage and frequency that correspond to the voltage and frequency of the local ac power network, then click the OK button to close the LVDAC-EMS Start-Up window.

6. In LVDAC-EMS, open the Four-Quadrant Dynamometer/Power Supply window. In the Tools menu of this window, select Friction Compensation Calibration, which will bring up the Friction Compensation Calibration dialog box. Click OK in this box to start the calibration process. Observe that the prime mover starts to rotate at high speed, thereby driving the shaft of the Four-Pole Squirrel Cage Induction Motor. The prime mover speed is then automatically decreased by steps to perform the calibration process. Once the calibration process is completed (which takes about two minutes), the prime mover stops rotating, then the Friction Compensation Calibration dialog box indicates that the calibration process is finished. Click OK in the Friction Compensation Calibration dialog box to close this box. Restart the Four-Quadrant Dynamometer/Power Supply to apply the changes (i.e., the newly calibrated friction compensation curve) by setting the main power switch of this module to O (off), and then I (on).

7. Connect the equipment as shown in Figure 20. In this circuit, the mechanical load is implemented using the Four-Quadrant Dynamometer/Power Supply coupled to the Four-Pole Squirrel Cage Induction Motor. The range of current input I1 to be used on the Data Acquisition and Control Interface depends on your local ac power network (see table in Figure 20).

a Current input I4 of the Data Acquisition and Control Interface is used to monitor the motor current in order to protect the motor from overheating if excessive currents flow through the stator windings. The range of current to be used for current input I4 is 40 A.

Exercise 1 – DOL Starters and Soft Starters  Procedure

Local ac power network Range of Input I1

(A) Voltage

(V) Frequency

(Hz)

120 60 40

220 50 4

240 50 4

220 60 4

Figure 20. Direct-on-line starter.

8. In the Data Acquisition and Control Settings window of LVDAC-EMS, set the Range of current input I1 as indicated in the table of Figure 20 for your local ac power network.

9. On the Synchronizing Module/Three-Phase Contactor module, set the Sync. switch to the O (off) position.

Control signal from DACI (DO1)

Remote control input

Three-phase contactor

Induction motor

40 A

Mech. load

Exercise 1 – DOL Starters and Soft Starters  Procedure

26 © Festo Didactic 88197-00

10. Connect Digital Output 1 (DO1) and a digital (D) common (white terminal) of the Data Acquisition and Control Interface to the + and – Remote Control terminals of the Synchronizing Module/Three-Phase Contactor module, respectively, using two miniature banana plug leads. These connections allow the three-phase contactor to be controlled using the Direct-On-Line Starter control function in LVDAC-EMS (via the Data Acquisition and Control Interface).

11. Connect Analog Outputs T and n of the Four-Quadrant Dynamometer/Power Supply to Analog Inputs 7/T and 8/n of the Data Acquisition and Control Interface, respectively, using two miniature banana plug leads. Connect the common (white terminal) of the Analog Outputs on the Four-Quadrant Dynamometer/Power Supply to an analog (A) common (white terminal) of the Data Acquisition and Control Interface. These connections are required to observe the induction motor speed and torque using the Oscilloscope.

12. In LVDAC-EMS, open the Four-Quadrant Dynamometer/Power Supply window, then make the following settings:

 Set the Function parameter to Mechanical Load. This makes the Four-Quadrant Dynamometer/Power Supply operate like a configurable mechanical load.

 Set the Load Type parameter to Flywheel. This makes the mechanical load emulate a flywheel.

 Set the Inertia parameter to 0.09 kg∙m2 (2.14 lb·ft2). This sets the moment of inertia of the emulated flywheel.

 Set the Friction Torque parameter to 0.1 N·m (0.88 lbf·in). This sets the torque that opposes rotation of the emulated flywheel.

 Set the Pulley Ratio parameter to 24:24.

a Note that the pulley ratio between the Four-Quadrant Power Supply/Dynamometer and the Four-Pole Squirrel Cage Induction Motor is 24:24.

13. In LVDAC-EMS, open the Thyristor Control window, then make the following settings:

 Set the Function parameter to Direct On-Line Starter.

 Set the Motor Full Load Current [FLA] parameter to the value of the motor rated current. The motor rated current is indicated on the motor front panel; it depends on the frequency and voltage of the local ac power network.

 Set the Overload parameter to On.

 Set the Overload Class parameter to 10.

Exercise 1 – DOL Starters and Soft Starters  Procedure

14. In LVDAC-EMS, open the Oscilloscope window. Make the appropriate settings to display one of the motor currents (measured via input I1), the motor torque (measured via Analog Input 7/T) and the motor speed (measured via Analog Input 8/n) using channels 1, 2, and 3, respectively. Set the vertical sensitivity of channel 1 to 2 A/div if your ac power network voltage is 120 V. Otherwise set the sensitivity of channel 1 to 1 A/div. Set the vertical sensitivity of channel 2 to 1 N·m/div (10 lbf·in/div). Set the vertical sensitivity of channel 3 to 500 (r/min)/div. Set the time base to 2 s/div. Finally, set the acquisition mode to Peak Detect.

Starting an induction motor using a DOL starter

In this part of the exercise, you will start the motor using a DOL starter. You will record the motor current, torque, and speed at start-up and analyze the results.

15. On the Power Supply, turn the three-phase ac power source on.

16. In the Four-Quadrant Dynamometer/Power Supply window, start the mechanical load by setting the Status parameter to Started or by clicking the Start/Stop button. The induction motor is now coupled to a flywheel emulated by the mechanical load.

17. In the Oscilloscope window, click the Single Refresh button, then immediately press the Start button of the DOL starter in the Thyristor Control window to start the motor.

a The above manipulation must be performed rapidly to ensure that the Oscilloscope records the entire evolution of the various parameters during motor start-up.

18. Wait until the Oscilloscope displays the evolution of the motor current, torque, and speed, then press the Stop button of the DOL starter to stop the motor.

a Since the Oscilloscope operates in peak detect mode and a slow time base is used, it displays the envelope of the motor current rather than its waveform.

19. In the Four-Quadrant Dynamometer/Power Supply window, stop the mechanical load by setting the Status parameter to Stopped or by clicking the Start/Stop button.

On the Power Supply, turn the three-phase ac power source off.

20. From the File menu of the Oscilloscope window, save to a text file (export) the data related to the motor current, torque, and speed displayed on the Oscilloscope screen.

Exercise 1 – DOL Starters and Soft Starters  Procedure

28 © Festo Didactic 88197-00

21. Using a spreadsheet software, plot the evolution of the motor current, torque, and speed as a function of time on three separate graphs:

22. From the motor current graph you plotted in the previous step, determine the maximal value that the motor current reached during motor start-up.

23. Compare the maximal current from step 22 to the motor FLA.

a The maximal current obtained in step 22 is a peak value. Convert this peak value to an rms value before making the comparison.

24. From the motor speed graph you plotted in step 21, is the maximal speed reached by the motor higher than the motor synchronous speed ( )?

 Yes  No

25. From the motor torque graph you plotted in step 21, determine the maximal torque that the motor produced during start-up.

26. Compare the maximal torque from step 25 to the motor nominal torque. The value of the motor nominal torque depends on the voltage and frequency of your local ac power network (see table below.)

Table 2. Nominal torque of the Four-Pole Squirrel Cage Induction Motor.

Local ac power network

Motor nominal torque T Voltage

(V) Frequency

(Hz)

120 60 1.13 N∙m (10.00 lbf∙in)

220 50 1.42 N∙m (12.56 lbf∙in)

240 50 1.40 N∙m (12.39 lbf∙in)

220 60 1.15 N∙m (10.18 lbf∙in)

Exercise 1 – DOL Starters and Soft Starters  Procedure

27. Would doubling the inertia of the mechanical load double the maximal torque produced during motor start-up? Why?

28. How would doubling the inertia of the mechanical load affect the start-up time?

29. From the motor speed graph you plotted in step 21, how long does it take for the motor to accelerate up to full speed?

Soft starter setup and settings

In this section, you will modify the equipment connections so as to replace the DOL starter with a soft starter.

30. Make sure that the three-phase ac power source is off.

31. Modify the equipment connections so as to obtain the circuit shown in Figure 21. The range of current input I1 to be used on the Data Acquisition and Control Interface depends on your local ac power network (see table in Figure 21).

Exercise 1 – DOL Starters and Soft Starters  Procedure

30 © Festo Didactic 88197-00

Local ac power network Range of Input I1

(A) Voltage

(V) Frequency

(Hz)

120 60 40

220 50 4

240 50 4

220 60 4

Figure 21. Soft starter setup.

Control signal from DACI (DO1)

Remote control input

Three-phase contactor control circuit

Induction motor

To thyristor gates

Firing control signals from DACI

Mech. load

40 A

Exercise 1 – DOL Starters and Soft Starters  Procedure

32. On the Synchronizing Module/Three-Phase Contactor module, make sure the Sync. switch is still set to the O (off) position.

33. Make sure that Digital Output 1 of the Data Acquisition and Control Interface is still connected to the Remote Control input of the Synchronizing Module/Three-Phase Contactor module.

34. Make sure that the T and n Analog Outputs of the Four-Quadrant Dynamometer/Power Supply are still connected to Analog Inputs 7/T and 8/n of the Data Acquisition and Control Interface.

35. Connect the Digital Outputs of the Data Acquisition and Control Interface to the Firing Control Inputs of the Power Thyristors module, using the provided cable with DB9 connectors.

36. In the Data Acquisition and Control Settings window of LVDAC-EMS, make sure that the Range of current input I1 is set as indicated in the table of Figure 21 for your local ac power network.

In the Four-Quadrant Dynamometer/Power Supply window, keep the settings of the Mechanical Load unchanged (i.e., as indicated in step 12).

In the Thyristor Control window, make the following settings:

 Set the Function parameter to Soft Starter.

 Set the Mode parameter to Soft Start.

 Set the Initial Torque parameter to 35%. The soft starter function uses this parameter to determine the voltage it must apply to the motor winding in order to obtain the desired initial torque.

 Set the Start Time parameter to 15 s. The start time is the period of time during which the Soft Starter function varies the voltage from an initial voltage (determined by the Initial Torque parameter) to the full voltage.

 Set the Kick Start Time parameter to 0.0 s to disable the kick start function.

 Set the Soft Stop parameter to 0 to disable the soft stop function.

 Set the Overload parameter to On.

 Set the Overload Class parameter to 10.

Exercise 1 – DOL Starters and Soft Starters  Procedure

Starting an induction motor using a soft starter

In this part of the exercise, you will start the motor, still coupled to the same mechanical load used earlier in the exercise, using a soft starter. You will record the motor current, torque, and speed at start-up and analyze the results.

37. On the Power Supply, turn the three-phase ac power source on.

38. In the Four-Quadrant Dynamometer/Power Supply window, start the mechanical load.

39. In the Oscilloscope window, click the Single Refresh button, then immediately switch to the Thyristor Control window and press the Start button of the soft starter to start the motor.

a The above manipulation must be performed rapidly to ensure the Oscilloscope records the entire evolution of the various parameters during motor start-up.

40. Wait until the Oscilloscope displays the evolution of the motor current, torque, and speed, and then press the Stop button of the soft starter to stop the motor.

41. In the Four-Quadrant Dynamometer/Power Supply window, stop the mechanical load.

42. On the Power Supply, turn the three-phase ac power source off.

43. From the File menu of the Oscilloscope window, save to a text file (export) the data related to the motor current, torque, and speed displayed on the Oscilloscope screen.

44. Using a spreadsheet software, plot the evolution of the motor current, torque, and speed as a function of time on three separate graphs:

45. From the motor current graph you plotted in step 44, determine the maximal value that the motor current reached during motor start-up.

46. From the motor torque graph you plotted in step 44, determine the maximal torque that the motor produced during start-up.

Exercise 1 – DOL Starters and Soft Starters  Procedure

47. Compare the maximal torque from step 46 to the motor nominal torque. The value of the motor nominal torque depends on the voltage and frequency of your local ac power network (see Table 2).

48. From the motor speed graph you plotted in step 44, how long does it take for the motor to accelerate up to full speed?

DOL starter versus soft starter

In this part of the exercise, you will compare the results you obtained with the DOL starter and the soft starter (for the same mechanical load) and try to deduce what the advantages and limitations of each one are.

49. Compare the current-versus-time curves obtained with both starters. Does using a soft starter significantly reduce the value of the current that the motor draws from the power source during start-up?

50. Compare the torque-versus-time curves obtained with both starters. Does using a soft starter significantly reduce the amount of torque that the motor produces during start-up?

51. Which motor starter makes the mechanical load accelerate more rapidly?

Exercise 1 – DOL Starters and Soft Starters  Conclusion

52. Summarize your results by checking the appropriate box:

The starter resulting in the lowest starting current is

 the DOL starter  the soft starter

The starter resulting in the lowest starting torque is

 the DOL starter  the soft starter

The starter providing the best acceleration is

 the DOL starter  the soft starter

53. In the Tools menu of the Four-Quadrant Dynamometer/Power Supply window, select Reset to Default Friction Compensation. This will bring up the Reset Friction Compensation dialog box. Click Yes in this window to reset the friction compensation to the factory default compensation.

54. Close LVDAC-EMS and then turn off all the equipment. Disconnect all leads and return them to their storage location.

In this exercise, you learned fundamental notions of motor starting. Namely, you learned about various phenomena occurring during motor start-up, motor protections, direct-on-line starters, and soft starters. In the exercise procedure, you wired two types of motor starters (a DOL starter and a soft starter) from basic components like a contactor and thyristors. You analyzed the start-up phenomena for both your DOL starter setup and soft starter setup. At the end of this exercise, you should have acquired a general understanding of DOL starters and soft starters and be able to point out the main differences between these two types of starter.

1. Identify the type(s) of motor starters that require an overload protection and explain why.

2. Explain what an inrush current is, as related to an induction motor.

CONCLUSION

REVIEW QUESTIONS

Exercise 1 – DOL Starters and Soft Starters  Review Questions

3. How does using a soft starter instead of a DOL starter to start an induction motor affect the motor starting current and torque, and the motor acceleration?

4. Explain how a soft starter can help reduce voltage sags.

5. An induction motor drives a conveyor belt via a chain and pulleys. A DOL starter is used to start the motor. The chain breaks frequently when the conveyor is started and the maintenance costs are high. Identify a possible cause for this problem and devise a solution to address this issue.